Method for monitoring adjacent zones in a mobile radio telephone system and a corresponding mobile radio telephone system

ABSTRACT

A method and system are provided for monitoring adjacent cells in a mobile radio telephone system, in which a mobile station located in a cell of a mobile radio telephone system receives, for purposes of monitoring adjacent cells, transmits signals coded with different cell-specific codes from base stations of adjacent cells and codes and evaluates these in accordance with the respective cell-specific codes, wherein at least the base station currently responsible for the mobile station transmits a signaling information item which informs the mobile station of the codes to be used in the monitoring of adjacent cells and/or of the codes not to be used in monitoring adjacent cells, and the mobile station, in monitoring adjacent cells, only decodes and evaluates the transmit signals of the adjacent cells coded with a code which can be used for the monitoring of adjacent cells in accordance with the signaling information.

[0001] The present invention relates to a method for monitoring adjacentcells in a mobile radio telephone system as claimed in the preamble ofclaim 1 and to a correspondingly designed mobile radio telephone system.

[0002] It is known that conventional mobile radio telephone systems areof cellular configuration, i.e. the respective mobile radio telephonesystem is formed by a multiplicity of radio cells and a base station isallocated to each radio cell. If a mobile station, for example a mobiletelephone, is located in the geographic area of a radio cell,communication with the mobile station within the mobile radio telephonesystem takes place between the mobile station and the base stationallocated to the current cell. If the mobile station is moving from onecell into another cell, the communication link existing between themobile station and the base station allocated to the “old” cell must behanded over to the base station allocated to the “new” cell. Thisprocess is called “handover”.

[0003] So as such a handover can be performed as rapidly and as free ofinterference as possible, it is necessary that the mobile stationlocated in the geographic area of a cell continuously monitors andevaluates the signals transmitted by the base stations of adjacent cellsin order to obtain in this manner system information of the adjacentcells which can be used for setting up communication between the basestation of the new cell and the mobile station as rapidly as possiblewhen entering into an adjacent new cell.

[0004] This monitoring of adjacent cells by the mobile station is mademore difficult due to the fact that the base stations in the individualcells use different scrambling codes. For the mobile station located ina cell, this means that it must be continuously adjusted for decodingand evaluating the transmit signals from base stations of adjacent cellsin accordance with the different scrambling codes. The more adjacentcells there are, the longer the monitoring of adjacent cells will take.

[0005] At national boundaries which, as a rule, also representseparating lines between two different mobile radio telephone networkoperators, the case may occur that the two network operators ofdifferent nationalities use the same frequency band. If a mobile stationof one network operator is close enough to the border of the othernetwork operator, the mobile station can receive the transmit signal ofthe cell of the other network operator on the other side of the border.In UMTS (Universal Mobile Telecommunication System) systems, forexample, the signals received by the mobile station are unambiguouslyallocated to a cell or to a network operator by reading a systeminformation item which is transmitted via the so-called broadcastcontrol channel (BCCH) of the base station of the respective cell.

[0006] Reading the system information item of an adjacent cell is atime-consuming process so that, for example, at a national boundary, thecase may occur that there is only one adjacent cell of the same operatorfor a cell in which the mobile station is currently located whereasthere are several adjacent cells of the other network operator so thatthe system information items of the adjoining cells of the other networkoperator must be read and evaluated during the monitoring of adjacentcells. In other words, the mobile station needs a large amount of timefor identifying the signal received from a network operator.Cumulatively, this time can become very large if there is a very largenumber of adjacent cells of a foreign network operator transmitting atthe same frequency as one's own network operator, i.e. if a very largenumber of adjacent cells of a foreign network operator are visible atthe same frequency for the mobile station.

[0007] To illustrate, a dashed national boundary is shown in FIG. 5which separates a cellular mobile radio telephone network of a networkoperator A and a cellular mobile radio telephone network of a networkoperator B. According to FIG. 5, the mobile radio telephone network Acomprises individual cells A0-A6 whereas the mobile radio telephonenetwork B comprises individual cells B0-B6. If, for example, a mobilestation of network operator A is located in cell A6, the mobile stationcan monitor both the adjacent cells A3 and A4 of its own mobile radiotelephone network A and the adjacent cells B0, B3, B4 and B1 of theforeign mobile radio telephone network B, i.e. the mobile station readsboth system information of cells A3 and A4 and system information ofcells B0, B3, B4 and B1 even though this would actually only be requiredfor cells A3 and A4 since cells B0, B3, B4 and B1 are cells of a foreignmobile radio telephone network.

[0008] The present invention is based on the object of providing amethod for monitoring adjacent cells in a mobile radio telephone system,and a correspondingly designed mobile radio telephone system, by meansof which the time needed for monitoring adjacent cells can be reduced.

[0009] According to the invention, this object is achieved by a methodhaving the features of claim 1 and by a mobile radio telephone systemhaving the features of claim 12. The subclaims in each case definepreferred and advantageous embodiments of the present invention.

[0010] According to the invention, signaling information is transmittedto the mobile station located in a cell from the base station of thecorresponding cell, which signaling information informs the mobilestation about the scrambling codes to be used or not to be used duringthe monitoring of adjacent cells. In this manner, the transmit signalsof the adjacent cells, to be observed during the monitoring of adjacentcells, or, respectively, the corresponding scrambling codes can bedelimited and thus the monitoring of adjacent cells can be acceleratedin that the mobile station is only informed about the scrambling codesused by the adjacent cells of its own network operator and,respectively, the scrambling codes used by the adjacent cells of aforeign network operator are excluded from the monitoring of theadjacent cells. This assumes that the individual adjacent cells in eachcase use different scrambling codes which is necessary in the case ofadjacent network operators which use the same frequency, even inaccordance with the current state of the art, by using common advancedplanning of the scrambling codes used in the individual cells.

[0011] The present invention is not restricted to the preferred field ofapplication of the scrambling codes but can be applied generally to allpossible types of cell-specific codes by means of which the basestations of the individual cells generate a transmit signal containing,in particular, the system information of the corresponding cell, thepresent invention being applicable, in particular, in UMTS mobile radiotelephone networks.

[0012] The signaling information informing the mobile station of thecodes to be used or not to be used in the monitoring of adjacent cellsis preferably provided in the system information of the current cell. Inthis case, in principle, it would be sufficient if this signaling istransmitted in each case in the cells of a mobile radio telephonenetwork close to the border, i.e. in the cells immediately adjoininganother mobile radio telephone network.

[0013] So that it is not necessary to report all codes to be used or notto be used for the monitoring of adjacent cells with the aforementionedsignaling information, this information can also be transmitted incorrespondingly coded form in order to reduce the signaling expenditure.

[0014] Thus, for example, it is possible to use the signalinginformation to report only the boundaries of the area in which thescrambling codes to be used or not to be used for the monitoring ofadjacent cells are located. In general, all scrambling codes used in theadjacent cells can be combined in subsets and only an information itemdesignating the required subset can be transmitted so that, inprinciple, only a number for designating the required subspace or therequired subset needs to be transmitted in this exemplary embodiment.

[0015] Another exemplary embodiment which allows a relatively largeamount of freedom to the network operators when issuing the scramblingcodes and can be coded using relatively few bits provides mapping ofindividual decimal scrambling codes of the adjacent cells in binary formand selection of certain bits of the binary scrambling codes with theaid of a corresponding mask. If the bits of the individual scramblingcodes selected by the mask correspond to a corresponding pattern, thismeans for the mobile station that the corresponding scrambling codes areto be used or not to be used in monitoring the adjacent cells. In thistype of signaling, only the mask and the required pattern need to bereported to the mobile station in the form of system information.

[0016] According to a further exemplary embodiment of the presentinvention, it is proposed to use scrambling codes which are in each casealigned with the same compass directions in the individual mobile radiotelephone networks or countries. In particular, this relates to thescrambling codes used in the cells close to the border. In the interiorof the country, in contrast, arbitrary scrambling codes can be used. Thealignment of the scrambling codes oriented in accordance with thecompass directions can be effected, for example, in such a manner thatcertain bits of the scrambling codes to be used or not to be used formonitoring the adjacent cells are specified in each case at the cornersof the individual mobile radio telephone networks corresponding to thesame compass direction. Thus, for example, the last two bits of thescrambling codes to be used or not to be used for monitoring theadjacent cells can be specified in each case, for example, at thecorners of the individual mobile radio telephone networks whereas onlyone of these last two bits of the scrambling codes is specified in thecells close to the border between two corners of a mobile radiotelephone network, i.e. in a transition zone between two corners. Inthis manner, only the current geographic position of the mobile stationwithin the respective mobile radio telephone network needs to betransmitted for signaling the scrambling codes to be used or not to beused in monitoring the adjacent cells which only requires three bits.Depending on this information about its own current geographic position,the mobile station can draw direct conclusions about the scramblingcodes to be used in each case for monitoring the adjacent cells.

[0017] Using this exemplary embodiment, it is thus possible to reducethe time required for monitoring adjacent cells with particularly littlesignaling expenditure and, moreover, collisions can be reliably avoidedsince due to the identical alignment of the scrambling codes inaccordance with the compass directions, provided in the individualadjacent mobile radio telephone networks or countries, the samescrambling codes cannot be used in adjacent cells or different networks.

[0018] In the text which follows, the present invention will beexplained in greater detail by means of preferred exemplary embodimentsand with reference to the attached drawing, in which:

[0019]FIG. 1 shows a representation for explaining by means of anexemplary embodiment the principle forming the basis of the presentinvention,

[0020]FIG. 2 shows a representation for explaining a further exemplaryembodiment of the present invention,

[0021]FIGS. 3 and 4 show representations for explaining a furtherexemplary embodiment of the present invention, and

[0022]FIG. 5 shows a representation for explaining the problem formingthe basis of the present invention.

[0023] In FIG. 1, two mobile radio telephone networks of a networkoperator A and of a network operator B, separated by a national border,are shown, mobile radio telephone network A comprising radio cells A0-A3and mobile radio telephone network B comprising radio cells B0-B2. Toeach cell, a base station is allocated which is responsible forcommunication with a mobile station within the geographic area of thecorresponding cell. FIG. 1 only shows the corresponding base station BSfor cell A0. It is assumed that a mobile station MS is currently locatedin cell A0.

[0024] In each cell, the corresponding base station uses anotherscrambling code, assuming that the use of scrambling codes of the twoadjoining network operators A and B has been jointly planned. FIG. 1shows the scrambling codes used by the corresponding cells in the formSC(n), where SC(n) is the scrambling code with the ordinal number n.

[0025] Each base station of the individual cells, particularly the basestations of cells A0, A1 and A3 and, respectively, B0-B2, which areclose to the border, have a list of the scrambling codes used in theadjacent cells of the same network operator and the scrambling codesused in adjacent cells of the adjoining network operator. In the exampleshown in FIG. 1, this means for cell A0 or the corresponding basestation BS that in this list, scrambling codes n=(12, 62, 112) aremarked as scrambling codes of its own network operator and scramblingcodes n=(39, 43, 57) are marked as scrambling codes of a foreign networkoperator. Transmitting this list in the form of corresponding signalinginformation to the mobile station MS located in the geographic area ofcell A0 informs the mobile station MS that only the scrambling codeswith n=(12, 62, 112) are to be used for monitoring adjacent cells andscrambling codes with n=(39, 43, 57) are not to be used for monitoringadjacent cells.

[0026] Depending on the network topology, however, this list with own orforeign scrambling codes can become relatively long so that thesignaling complexity can also correspondingly become relativelyextensive. In the text which follows, therefore, exemplary embodimentsare shown in which signaling of the scrambling codes to be used or notto be used for monitoring adjacent cells is possible with reducedexpenditure.

[0027] Thus, for example, it is possible to use the signalinginformation transmitted from the base station BS of cell A0 to mobilestation MS to report only an area in which the scrambling codes to beused or not to be used for monitoring adjacent cells are located. Thus,it is only necessary to transmit two numbers, namely the beginning andthe end of the corresponding area, to the mobile station MS. For theexample shown in FIG. 1, this means that the base station BS only needsto transmit the area boundaries “39” and “57” to the mobile station MSin order to report the scrambling codes to be used for monitoringadjacent cells.

[0028] A further possibility for signaling with little expenditure issubdividing the scrambling codes used in the adjacent cells into anumber of subspaces or subsets so that it is only necessary to transmita number for designating the subset with the scrambling codes requiredfor monitoring adjacent cells to the mobile station MS. In the exampleshown in FIG. 1, for example, the scrambling codes of the cells A1-A3 ofthe same mobile radio telephone network A, which are adjacent to cellA0, and the scrambling codes of the cells B0-B2, which are adjacent tocell A0, of the foreign mobile radio telephone network can be combinedin subsets in such a manner that the scrambling codes of the adjacentcells of their own mobile radio telephone network A all have theproperty n=even where the scrambling codes of the adjacent cells of theforeign mobile radio telephone network B have the property n=odd.

[0029] At points at which more than two different mobile radio telephonenetworks meet one another, this dividing of the scrambling codes can becontinued where, for example, the scrambling codes of the adjacent cellsof their own mobile radio telephone network A generally meet theproperty n mod p, p being the number of meeting mobile radio telephonenetworks.

[0030]FIG. 2 shows a further exemplary embodiment of the presentinvention for signaling the scrambling codes to be used in monitoringadjacent cells with little expenditure, the example shown in FIG. 2allowing a relatively large amount of freedom to the network operatorsin issuing the scrambling codes and being codeable with a relativelysmall number of bits.

[0031]FIG. 2 shows the individual scrambling codes shown in FIG. 1 andthe corresponding ordinal numbers of the cells adjacent to cell A0 bothin decimal and in binary form. A mask m is defined which only selectscertain bits of the individual binary scrambling codes. If the bitsselected by the mask m form a particular pattern, the pattern “011” inthe exemplary embodiment shown in FIG. 2, this means for the mobilestation MS that the corresponding scrambling code is not to be used inmonitoring adjacent cells.

[0032] In the exemplary embodiment shown in FIG. 2, with m of length 8bits, it is only necessary to specify the mask m with 8 bits and thepattern with 3 bits to be monitored for coding the system information.The information of the mask m or, respectively, of the correspondingrule, which 3 bits are to be selected from the 8 bits of the ordinalnumber n of the individual scrambling codes, could be coded with only 6bits since there are only 56 possibilities of selecting three bits fromeight bits.

[0033]FIG. 3 and FIG. 4 show a further exemplary embodiment of thepresent invention which enables the time required for monitoringadjacent cells to be shortened with particularly low signalingexpenditure.

[0034] The assumption is here that the scrambling codes used in theindividual cells are defined in accordance with the arrangement shown inFIG. 3 in all mobile radio telephone networks or countries, particularlyin the mobile radio telephone networks or countries adjoining oneanother. FIG. 3 always indicates by way of example the allocation of thelast two bits of the binary scrambling codes or, respectively, a numberassociated with this allocation, the individual rectangles in each casecomprising groups of cells.

[0035] As is shown in FIG. 3, various scrambling codes are permittedwithin a country or mobile radio telephone network since the last twobits of these scrambling codes are occupied by “xx”. Whereas theassignment is arbitrary in the interior of the country, the last twobits of the scrambling codes used in the corresponding cells arespecified in the four corners of the country or mobile radio telephonenetwork, respectively. Thus, for example, only scrambling codes the lasttwo bits of which are occupied by “00” are used in all cellscorresponding to the north-eastern corner, etc. For the edge or borderareas, in contrast, only one of these bits is specified so that, forexample, only scrambling codes, the last-but-one bit of which isoccupied by “0” are used for the cells which are located close to theborder between the north-eastern corner and the southeastern corner.

[0036] If the mobile radio telephone network or country considered ineach case does not have a rectangular shape according to FIG. 3, theprinciple must be correspondingly generalized.

[0037] Naturally, a different association between the directions andpossible scrambling codes is also possible, e.g. the arrangementaccording to FIG. 3 can be rotated and/or mirrored or it can begeneralized to a larger number of directions, (e.g. 8 directionsspecified by 3 bits of the scrambling code).

[0038] To inform the mobile station, which, for example, is located in acell close to the border, of the scrambling codes to be used formonitoring adjacent cells, it is only necessary to inform the mobilestation of an identifier designating the current geographic position ofthe mobile station within the corresponding mobile radio telephonenetwork. Since, according to the arrangement shown in FIG. 3, adistinction is made between eight different compass directions, 3-bitsignaling is adequate for this. If, for example, the mobile station isinformed in this way that it is located in the north-eastern corner ofthe corresponding mobile radio telephone network, the mobile station canderive from this fact that it should only use scrambling codes the lasttwo bits of which are occupied by “00” for monitoring adjacent cells.All other scrambling codes are not used by the mobile station formonitoring adjacent cells.

[0039]FIG. 5 shows for the example of European mobile radio telephonenetworks or, respectively, European mobile radio telephone networkoperators how the arrangement shown in FIG. 4 can be mapped onto theindividual countries or mobile radio telephone networks. In each case,the north-eastern corner is designated by “0”, the south-eastern cornerby “1”, the north-western corner by “3” and the south-western corner by“2” (compare also FIG. 3).

[0040] As can also be seen from FIG. 4, it is not possible forcollisions to occur, i.e. it is possible for the same scrambling codesto be used in adjacent cells of different countries or mobile radiotelephone network operators since the same arrangement (compare FIG. 3)is used in all countries or mobile radio telephone networks forallocating or specifying the scrambling codes, and a national border orcorner at which a number of countries meet cannot be located in the samecompass direction seen from all the countries. For example, a pointcannot be interpreted at the same time as the south-eastern corner bytwo (or more) adjacent countries or a border cannot be simultaneouslythe southern border of two countries.

1. A method for monitoring adjacent cells in a mobile radio telephonesystem, in which a mobile station (MS) located in a cell of a mobileradio telephone system receives, for the purpose of monitoring adjacentcells, transmit signals coded with different cell-specific codes frombase stations (BS) of adjacent cells and codes and evaluates these inaccordance with the respective cell-specific codes, characterized inthat at least the base station (BS) currently responsible for the mobilestation (MS) transmits a signaling information item which informs themobile station (MS) of the codes to be used in the monitoring ofadjacent cells and/or of the codes not to be used in monitoring adjacentcells, and in that the mobile station (MS), in monitoring adjacentcells, only decodes and evaluates the transmit signals of the adjacentcells coded with a code which can be used for the monitoring of adjacentcells in accordance with the signaling information.
 2. The method asclaimed in claim 1, characterized in that the codes are scramblingcodes.
 3. The method as claimed in claim 1 or 2, characterized in thatat least the base stations (BS) of the cells arranged at a borderbetween one mobile radio telephone network telephone operator (A) andanother mobile radio telephone network operator (B) transmit thesignaling information, in which the codes, to be used in accordance withthe signaling information for monitoring adjacent cells, correspond toadjacent cells of the same network operator (A) or, respectively, thecodes not to be used according to the signaling information formonitoring adjacent cells correspond to adjacent cells of the othernetwork operator (B).
 4. The method as claimed in one of claims 1-3,characterized in that the signaling information is transmitted in theform of system information.
 5. The method as claimed in one of thepreceding claims, characterized in that each base station (BS)transmitting the signaling information has a list about the codes to beused in monitoring the adjacent cells and/or a list about the codes notto be used in monitoring the adjacent cells.
 6. The method as claimed inone of the preceding claims, characterized in that the signalinginformation informs the mobile station (MS) of an area in which thecodes to be used or not to be used for monitoring the adjacent cells arelocated.
 7. The method as claimed in one of claims 1-5, characterized inthat the codes used in the adjacent cells are combined in subsets, onesubset comprising the codes to be used in monitoring the adjacent cells,and in that the signaling information informs the mobile station (MS) ofthe subset with the code to be used in monitoring the adjacent cells. 8.The method as claimed in one of claims 1-5, characterized in that thesignaling information informs the mobile station (MS) of a particularmask and of a particular bit pattern, the mask being used for selectingcertain bits from the binary representation of the individual codes ofthe adjacent cells, and that the mobile station uses for the monitoringof adjacent cells only codes the bits of which, selected by the mask,either correspond to the particular bit pattern or do not correspond tothe particular bit pattern.
 9. The method as claimed in one of claims1-5, characterized in that in the individual cells of the mobile radiotelephone system (A, B), the codes used by the respective base station(BS) are defined in accordance with the geographic position of therespective cell in the mobile radio telephone system (A, B), and in thatthe signaling information informs the mobile station (MS) of the currentgeographic position of the mobile station (MS) within the mobile radiotelephone system (A, B), the mobile station (MS) drawing conclusionsabout the codes used in the adjacent cells from this information aboutits own geographic position within the mobile radio telephone system (A,B) and using these codes for monitoring the adjacent cells.
 10. Themethod as claimed in claim 9, characterized in that, for cells arrangedwithin the same geographic area of the mobile radio telephone system (A,B), at least one bit of the code used in the corresponding cells isidentically specified, two bits being identically specified particularlyfor cells located in the corner areas of the mobile radio telephonesystem (A, B) whereas at least one bit of the codes used in the cells isidentically specified for cells of the mobile radio telephone system (A,B) arranged in edge areas with one another.
 11. The method as claimed inclaim 9 or 10, characterized in that a number of adjacent mobile radiotelephone systems (A, B) are provided and in that in each mobile radiotelephone system (A, B), the codes used in the corresponding cells aredefined in accordance with the same arrangement in accordance with thegeographic position of the respective cell in the respective mobileradio telephone system (A, B).
 12. A mobile radio telephone system, themobile radio telephone system (A, B) comprising a number of cells (A0,A3, B0-B2), each cell being associated with a base station (BS) whichgenerates a transmit signal coded with a particular cell-specific code,and a mobile station (MS) located in a particular cell of the mobileradio telephone system (A, B), for monitoring adjacent cells, receivingthe transmit signals from the base stations (BS) of adjacent cells,decoding and evaluating them in accordance with the respectivecell-specific code, characterized in that at least the base station (BS)currently responsible for the mobile station (MS), of the cell in whichthe mobile station (MS) is currently located, is designed in such amanner that it transmits signaling information which informs the mobilestation (MS) of the codes to be used in monitoring the adjacent cellsand/or of the codes not to be used in monitoring the adjacent cells, andin that the mobile station (MS) is designed in such a manner that itreceives the signaling information, evaluates it and, in monitoring theadjacent cells, only decodes and evaluates the coded transmit signals ofthe adjacent cells which can be used for monitoring the adjacent cellsin accordance with the signaling information.
 13. The mobile radiotelephone system as claimed in claim 12, characterized in that the basestation (BS) and, respectively, the mobile station (MS) is designed forcarrying out the method as claimed in one of claims 1-11.
 14. The mobileradio telephone system as claimed in one of claims 12 or 13,characterized in that the mobile radio telephone system is a UMTS mobileradio telephone system.